Electrical transmission systems and filters therefor



Nov. 18, 1958 L. o. KRAUSE 2,861,245

ELECTRICAL TRANSMISSION SYSTEMS AND FILTERS THEREFOR Filed Sept. 26,1952 F ii. I. Y

MITTER Pi .4 ix 2 l 22 Pi .3. 2 ,8- i g 1 1 7 i 0/ si I I i i .fiatfz-z.o f, i I f2 i 2 l 2 a- E +0 i I i 7 fl 2 Inventor;

Lloyd 0. Krause His Attorney.

ELECTRICAL TRANSMISSION sYsTEMs AND FILTERS THEREFOR Lloyd 0. Krause,North Syracuse, N. Y., assignor to General Electric Company, acorporation of New York Application September 26, 1952, Serial No.311,760

10 Claims. (Cl. 333-9) This invention relates to high frequencyelectrical transmission systems, and more particularly, to systems forfeeding energy from two transmitters operating at dif ferent frequenciesto a common load, and to filters for such systems. Such systems arecommonly employed in television transmitter apparatus where energiesfrom a visual transmitter and an aural transmitter are simultaneouslyfed to a single antenna. Systems of this type are commonly referred toas diplexers.

The diplexer embodying this invention operates simultaneously to feedenergy from two transmitters to a common load through an intermediatecircuit comprising filter elements that are constructed and arrangedrelative the individual transmitters and the common load so that both 25transmitters may feed into the load and yet remain adequately decoupled.The principles of diplexer operation are well known and will not bedescribed in detail. My invention resides in certain improvedconstructional features of the diplexer assembly and in novel filterelements employed therein, whereby improved functioning of the assemblyis obtained.

One of the objects of this invention is to provide a new and improvedsystem for transmitting energy generated by a pair of transmitters to asingle antenna in such a manner that there is no reaction between thetransmitters.

A further object of this invention is to provide a novel diplexerembodying filter apparatus that is characterized in that substantiallyzero attenuation is effected at a first frequency value andsubstantially infinite attenuation is effected at a second frequencyvalue.

Still another object is to provide a compact coaxialline filter elementthat is sharply resonant at a first frequency value and also sharplyantiresonant at a second frequency value, said frequency values beingspaced a small predetermined amount in the spectrum of high frequencies.

Another object is to provide a filter apparatus of the type hereinbeforedescribed that is simple and economical to produce.

In carrying out the invention in one form, there is. provided a systemincorporating novel filter elements in the form of coaxial transmissionline sections each comprising a tandem arrangement of a first sectionhaving a relatively low characteristic impedance and a second sectionhaving a relatively high characteristic impedance, the electrical lengthof the low-characteristic impedance section being of the order of A/ 8,and the electrical length of the high-characteristic impedance sectionbeing of the order of Ill, 4

where A is the wavelength at the mean frequency of the design range ofthe diplexer and n is an integer.

Other objects will become apparent and the invention better understoodfrom a consideration of the following description taken in conjunctionwith the accompanying drawing in which:

Fig. 1 is a block diagram of an electrical transmission system inaccordance with one embodiment of the invention; i V

Fig. 2 is a longitudinal cross sectional view of a filter elementconstructed according to the principles of one aspect of the invention;and

Figs. 3 and 4 are graphs explanatory of the operation of the invention.

Referring to Fig. 1, there is shown, in block -diagram' ,form, anelectrical transmission system embodying the novel feature of theinvention. The system comprises a transmission line 11, which may be ofthe well-known coaxial, waveguide or parallel-wire type, extendingbetween a visual television transmitter 13 operable at a frequency f andan aural television transmitter 15 oper- 15 to current practice, thevisual frequency is lower than and spaced from the frequency at whichthe aural transmitter operates by approximately 4.5 megacycles persecond.

Modulation components of h, the frequency of the visual transmitter 13,usually extends to 4 megacycles or .5

megacycles below f which is about /2 at the frequency 13 and the auraltransmitter 15, (preventing the signalvalues currently employed bycommercial telecasting. It will, of course, be understood that thevalues here indicated are by way of example only and are not intended aslimitations or restrictions. 7

The signals from the visual transmitter 13 and aural transmitter 15 areapplied to a common antenna 17, which is coupled to the transmissionline 11 by means of a line section 19 forming a junction 21 with theline 11.

For minimizing reaction between the visual transmitter from onetransmitter from reaching the other transmitter) and at the same timeproviding a low-impedance path for the signals from each transmitter tothe antenna 17, a pair of filter elements 23 and 25 is provided,(indicated by rectangles), one of which, say filter element 23, providesa termination for a quarter-wave section 27 of coaxial transmission lineconnected to the transmission line 11 at a point in the visual armthereof one quarter wavelength from the junction 21. The other filterelement 25 provides a termination for a half-wave section 29 of coaxialtransmission line connected to the transmission line 11 at a point inthe aural arm thereof one quarter Wavelength from the junction 21. Thedistances expressed herein, in terms of fractions of a wavelength, mayfor practical purposes refer to the wavelength at the mean frequency ofthe design range of the system. Further, these distances may vary aboutthese mean values for purposes of compensation. Eachof the filterelements 23 and 25, as will be mor fully explained hereinbelow, isadapted to be series resonant at the frequency f of the visualtransmitter 13 and parallel resonant at the frequency f of the auraltransmitter 15. Thus, since the filter element 23 terminates thequarter-wave section 27, which as is known, operates as animpedance-inverting element, the combined filter element 23 andquarter-wave section 27 present very high shunt impedance to signals ofthe visual frequency (f and very low, substantially zero, shuntimpedance to signals of the aural frequency (f with respect to thefilter element 25 that terminates the half-wave section 29, since ahalf-wave section of coaxial transmission line, as is well known,presents at its input the same impedance as seen at its termination.Thus,- the impedance to signals of the aural frequency (f2).

The ability of the stub-connected filters to effect substantiallycomplete separation between the signals from one transmitter and theother transmitter is dependent on the sharpness of the cut-offcharacteristic of the filter,

element itself.

The opposite is true I 2 illustrates a filter element constructedaccardina s with the principles of one aspect of this invention andhaving a desirably sharp cut-01f characteristic. The'filter element 31comprises a high impedance,

.- The endof the high-impedance section 33 remote fromtliellow-impedance section 35 is closed as by a conductive plate- A3short circuiting the conductive cylinder 37 to the first portion 39 ofthe coaxially mounted conductive (the inner and outer conductors of thehigh-im pedance section 33)., The low-impedance section ,35 isopencircuited at both ends. To facilitate connection of the filter element31 to the quarter-wave section 27,.

the open end of filter element 31 can be flanged, as at 42", forfastening to a similarly flanged end 44 of the quarter-Wave section 27.Also, the second portion 41 of the coaxially mounted conductive rod canbe formed with a projecting tip 46 that fits into a mating. slot inthe'inner conductor of the quarter-wave section 27 thus to; providesmooth electrical continuity at the junction of'the filter element 31and the quarter-wave section 27. The connection of a similar filterelement 31 to the halfwave section 29 may be likewise effected. Bydimensioning the high-impedance section 33 and the low-impedance section35 so that the latter is substantially an eighth wavelength at the meanfrequency of the design range; and the former is an effective integralnumber of half wavelengths at said mean frequency, the series-connectedsections provide, in effect, a capacitance-inductance L section inseries circuit with a series-resonant inductancecapacitance circuit. Atthe visual frequency (f the total series resonant circuit offers zeroimpedance. At

the aural frequency (f the seriescircuit and the L-sec-- tion" go intoparallel resonance and present substantially infinite input impedance.

To explain in greater detail the operation ofthe filter element 31whereby the desired extremely sharp cutotf characteristic is derived,reference is made to Figs". 3 and 4', which are graphs illustratingcertain: impedance relationships existent at various points of theelement; In these figures, the following symbols are used having thedefinitions indicated: 7

lg is:the characteristic impedance of thetransmiss'ion lines 11, thequarter-wave section 27 and the half-wave section 29 to which the filterelements 31 are connected.

Z is the impedanceseen lookinginto the low-im pedance section 35. I

Z is the characteristic impedance of the low-im pedance section 35.

Z is the impedance seen looking into the high im pedance section 33 atthe junction of the sections 33' and 35. i A

In Fig. 3, the line 45 is a graph depicting the slope of the 'reactanceprovided by the high-impedance section 3 3j, the ordinates of the graphbeing the ratio Z Z/Z 1 and the abscissa's being frequency. As shown;thereactance slope is selected so that the ratio Z /Z i is -'1"at thefre-" qdncy'f of the visual transmitter 13 and +1 at the frequency f ofthe aural transmitter 15. It will be understood that, although thisslope of reactanceis ab: tained, in the illustrated embodiment, by ashort-circuited section of transmission line ofeffective length equal'toan integral number of half wavelengths at the mean'fr e qiincy-of thedesign range of'the diplexer,"'such a slope which is the condition of'antiresonance.

i at thefrequericy fi-and' may alsobe obtained by open-circuitedsections of length hill 4 where m is an odd integer and A is the samemean frequency, or by proper coupling into a resonant cavity. Thesignificance of the reactance slope will be clear from a considerationof .the following analysis. From well-known transmission line theory, ina lossless line, and since low-impedance section is'of electrical lengthequal to 45 we have Pram-negates of Fig; '3; it will be sen that a so ,Z01 V at the frequency f and-the requirementsfor the' locati o'n "of theresonant and a'ntiresonant frequencies are'-thus"es-' tablishedh a Fig:4 illustrates the slope of the reactanee' in the region ofaural'frequen'cy. 3, curves 47 and'49 thereoflbe'ing the' plots ofthera'tio Z /Z against frequency and" Zf/Zty against Y frequency,respectively. Curve 49 illu'stlate'sWh reactance slope aftertrans'forl'natiou through the eighth" wavelength low-impedance section35; It 'will' be noted that resonance occursat thevisual frequency fa"-rid'-ari'ti'-' resonance occurs at the" aural frequency f since" thecurve passes throughzero at the-former frequencyaiid' to infinity at thelatter.-

For the investigation of the conditions in the aural frequency-f2 itiscohvenient'to consider 'th'e us eptance" instead'of th'e' reactan'cebecause the suseeptaaea -passes through zero at aural frequency arid inthis" I Normalizing Equation 3 with-respect to Z the impedance acrosswhich the element isconnected, by multiplying through by Z /Z we have Z,o. 01 4 I, v 0/ 1 n oi'i Z2 (4) Dimensions one respect to frequency; wehave f- Zs f (-Zm+zr za+a1 Nae; at anti nest-easy 5," the astiresaaaacefrequency? ZFZOI' .t hsa tf e frequency f and aural frequency f theresonant and antiresonant frequencies. Since, at visualfrequency f Z =Zand at aural frequency f Z =Z we have dz; 2Z0] f f2-f1 V (7)Substituting the value of a'Z /df from Equation 7 into Equation6 we haved(Z /Z .Z .1 8

f zm p-n which demonstrates that for a given fixed frequency separation,f f the normalized susceptance slope at is directly proportional to theratio e. g., the larger the value of the ratio, the steeper the slope.Or, by suitably selecting Z any desired slope may be obtained. This isgraphically shown by the curve 49, plotting as abscissas the reactance Z/Z instead of the susceptance 2 /2 for ease of comparison with curve 47.

It will thus be seen that the region around f may be termed a slotregion, the width of which can be controlled by selecting a suitablevalue of Z the characteristic impedance of the low-impedance section 35.

In an operative embodiment, an impedance characteristic as illustratedby the curve 49, obtained for the case where Z /Z =0.065 effectivelyshort-circuited the transmission line 11 (Fig. 1) except in the regionof the slot at aural frequency f It was found that a normalizedreactance of 0.5 at 4 megacycles per second above visual frequency f thefrequency width of the visual transmitter 13, is sufficiently small topermit a fiat pass characteristic for all the visual frequencies, byemploying compensation techniques.

The employment of filter elements of the kind described above in thesystem of Fig. 1 thus provides the desired steep slot for ensuringsubstantially complete separation between the signals from onetransmitter and the other transmitter.

A further advantage following from the use of the filter element of theinvention resides in the fact that while at aural frequency f acondition of antiresonance is obtained for the filter elements 23 and25, resonance is at the same time obtained at the visual frequency h.This results in a very high rejection for frequency values at and aroundvisual frequency 11, and since visual frequency f is generally thevisual carrier of a television signal, most of the energy isconcentrated thereabout. In a practical system it has been observed thatthis feature results in an additional rejection of about 30 decibelswith respect to the rejection heretofore obtained using priorknowncircuits only sharply antiresonant at the mean frequency of the auralcarrier.

.While a particular embodiment of this invention has been shown anddescribed, it will, of course, be understood that various modificationsmay be made without departing from the invention. Therefore, by theappended claims, it is intended to cover all such changes andmodifications as fall within the truespirit and scope of the invention.

What [claim as new and desire to secure by Letters Patent of the UnitedStates is: 7

l. A high-frequency wave filter for attenuating Waves of a firstfrequency value and transmitting waves of a second frequency value, saidtwo frequency values being spaced apart a small predetermined amount,comprising a first section of coaxial transmission line having anelectrical'length substantiallyiequal to an integral number of Now, dZ/df is determined by the location ofvisual halt Wavelengths at the meanvalue of said first andsec- 0nd frequency values and short circuited atone end thereof, and a second section of coaxial transmission line hav-.ing one end coupled to the other end of said first section and having anelectrical length substantially equal to ,an

odd number of eighth wavelengths at said mean frequency value, thecharacteristic impedance of said second section having a value such thatthe reactance characteristic of said first section as seen from theother'end of said second section is transformed into a resultantcharacteristic defining a resonance at said'first frequency value and anantiresonance at said second frequency value.

2. A high-frequency Wave filter having resonance atone frequency andantiresonance at another frequency closely spaced from said onefrequency comprising a section of transmission line substantiallyone-eighth of a wave length long at the mean of said frequencies, areactive impedance element connected across one end of said section,said reactive impedance element having zero reactance at said meanfrequency, a capacitive reactance at said one frequency and an inductivereactance at said other frequency, said capacitive reactance at said onefrequency and said inductive reactance at said other frequency eachbeing equal to the characteristic impedance of said section oftransmission line whereby at. the other end of said section appears alow impedance at said one frequency and ahigh impedance at said otherfrequency.

3. A high-frequency wave filter element for presenting a low impedanceto waves of a first'frequency and a high impedance to waves of a secondfrequency closely spaced from said first frequency, comprising a firstsection of.

transmission line having a length equal to an integral number of halfwave lengths at the mean of said frequencies and being shortcircuited atone end, a second section of transmission line having a length equalsubstantially to an odd number of substantially one-eighth wave lengthsat said mean value and having a character-' said second frequency eachbeing equal to the characteristic impedance of said section oftransmission line. I

4. A high-frequency wave filter system for transmitting with one degreeof effectiveness waves having frequencies lying in the vicinity of afirst frequency and for trans mitting with another degree ofeffectiveness waves having frequencies lying in the vicinity of a secondfrequency closely spaced from said first frequency, comprising a maintransmission line for carrying waves of said fre-' quencies, a sectionof transmission line substantially oneeighth ofa wave length long at themean of said first and second frequencies, said section of transmissionline having one end coupled to said main transmission line, a-

reactive impedance element connected across the other end of saldsection, said reactive impedance element having zero reactance at saidmean frequency, a capacitive. reactance at said first frequency and aninductive reactance at said second frequency, the capacitive reactanceat said first frequency being equal to the inductive reactance at:said-second frequency and each of said reactances being equal to thecharacteristic impedance of said section of transmission line.

5. A high-frequency wave filter system for transmitting 7 with onedegree of effectiveness waves having frequencies lying in the vicinityof a first frequency and for transmitting with another degreeeffectiveness waves having: frequencies lyingin the vicinity of a secondfrequency closely spaced from, said first frequency, comprising a marstransmission, line for carrying waves of said freqil'eric'iiesj asection" of transmission line substantially oneei'g hth of a wave lengthlong at the mean of said first andsecond frequencies, said section oftransmission line having one end coupled to said main transmission line,a reactive impedance element connected across the other end of saidsection, said reactive impedance element having Zero reactance at saidmean frequency, a capacitive Iea'cta'n ce at said first frequency and aninductive reactance at said second frequency, the magnitude of saidcapacitive reactance at said first frequency being equal to themagnitude ofsaid inductive reactance at said second ffequeneyand themagnitude'of each of said reactanccs being equal to the magnitude of thecharacteristic imenance of said section of transmission line, saidsection of tians't nission line having a characteristic impedancesubstantially less than the characteristic impedance of said maintransmission line.

6. A high-frequency wave filter system for passing waves over a broadband of frequencies in the vicinity of a first frequency and attenuatingWaves having frequeneies extending over a narrow band of frequencies inthe vicinity of a second frequency from said first frequeneycomprising amain transmission line for carrying waves having frequencies lying insaid bands of frequencies, a section of transmission line substantiallyoneeighth of a wave length long at the mean of said first and secondfrequencies, a reactive impedance element connected across one end ofsaid section, said reactive impedance element having zero reactance atsaid mean frequency, a capacitive reactance at said first frequency andan inductive reactance at said second frequency, the magnitude of saidcapacitive reactance at said first frequency and magnitude of saidinductive reactance at said second frequency each being equal to themagnitude ofthe characteristic impedance of said section of transmissionline, said section of transmission line having a characteristicimpedance substantially less than the characteristic impedance of themain transmission line, and animpedance transformer for coupling theother end of said one-eighth section in shunt with said maintransmission line.

7. A high-frequency wave filter system for rejecting waves over a broadband of frequencies in the vicinity ofa first frequency and transmittingwaves having frequencies extending over a narrow band of frequencies inthe ,vicinity of a second predetermined frequency closely spaced fromsaid first frequency, comprising a main transmission line for carryingwaves having frequencies lying in said bands of frequencies, a sectionof transmission line substantially one-eighth of a Wave length long atthe mean of said first and second frequencies, a reactive impedanceelement connected across one end of said section, said reactiveimpedance element having zero reactance at said mean frequency, acapacitive reactance at said first frequency and an inductive reactanceat said second frequency, the magnitude of said capacitive reactance atsaid first frequency and the magnitude of said inductive reactance atsaid second frequency each being equal to the magnitude of thecharacteristic impedance of said section of transmission line, saidsection of transmission line having a characteristic impedancesubstantially less than the characteristic impedance of said maintransmission line, a section of transmission line one-half ofawavelength long at said mean frequency having one endconnected inshunt withsaid main transmission line and the-otherend connected to the other endof said oneeighth wave length section.

81 A high-frequency wave filter system for rejecting waves-overabroadband of frequencies in the vicinity of a fi'istfrequency andtransmitting waves having freque 'ie's eritending' over a narrow band offrequencies in mevrei i of a secondpredetermined"frequency closelyspaced rraia said-arsrrrsquena comprising a main transmission line forcarrying waves having frequencies lying in said bands of frequencies, asection of transmission line substantially one-eighth of a wave. lengthlong at the mean of said first and second frequencies, a reactiveimpedance element connected across one end of said section, saidreactive impedance element having zero reactance at said mean frequency,a capacitive reactance at said first frequency and an inductivereactance at said second frequency, the magnitude of said capacitivereactance at said first frequency and the magnitude of said inductivereactance at said second frequency each being equal to the magnitude ofthe characteristic int pedance of said section of transmission line,said section of transmission line having a characteristic impedancesubstantially less than the characteristic impedance of said maintransmission line, a section of transmission line one-quarter wavelength long at said mean frequency having one end connected in shuntwith said main transmission line and the other endjconnected to theother end of said one-eighth wave length section.

9. A high-frequency wave filter element for presenting a low impedanceto waves of a first frequency and a high impedance to waves of a secondfrequency closelyspa ced from said first frequency, comprising a firstsection of transmission line having a length equal to an integral; oddnumber of quarter wave lengths at the mean of said frequencies and beingopen circuited at one end, a second section of transmission line havinga length equal substantially to an odd number of substantiallyone-eighth wave lengths at said mean value and having a character isticimpedance relatively low with respect to the characteristic impedance ofsaid first section of transmission line, one end of said second sectionbeing connected to the other end of said first section, thecharacteristic impe'd ance and length of said first section beingso'chosen sucli' that said first section of said transmission linepresents a capacitive reactance to said one end of the one-eighthsection at said first frequency and an inductive reactanc'e at saidsecond frequency, said capacitive reactance at said first frequency andsaid inductive reactance at said second frequency each being equal tothe characteristic impedance of said section of transmission line.

10. A high-frequency wave transmission system corn-f prising a source ofhigh-frequency waves, a' utilization means for high-frequency wav'esresponsive to said source of high-frequency waves, and a high-frequencywave filter having resonance at a first frequency and an'ti resonance ata second frequency closely spaced from said first frequency, interposedbetween said source of highfrequency Waves and said utilization means,saidliighf frequency wave filter comprising a section of trans'fm sion'line substantially one-eighth of a wavelength lo g: at the mean of saidfirst and second frequencies, a reactive impedance element connectedacross one end of saidsecf said first frequency and an inductiverea'ctancc at said second frequency, said capacitive reactance at saidfirst J frequency and said inductive reactance at said second frequencyeach being equal to the characteristic impedance of said section oftransmission line'whereby at the other end of said section oftransmission line appears low" impedance at one of said frequencies andhigh impe'dat'ice at the other of said frequencies.

References Cited in the file of this pat ent UNITED STATES PATENTS

